1. Field of the Invention
This invention relates to an electrostatic latent image read-out method and apparatus. This invention particularly relates to an electrostatic latent image read-out method and apparatus, wherein reading light having been produced by a reading light source is irradiated to an electrostatic recording material, on which image information has been recorded as an electrostatic latent image, and which is capable of outputting a signal in accordance with the electrostatic latent image by being subjected to exposure scanning with the reading light, and the electrostatic latent image is thereby read out from the electrostatic recording material.
2. Description of the Related Art
Systems for recording and reading out radiation image information by the utilization of electrostatic recording materials have heretofore been proposed. With the proposed systems for recording and reading out radiation image information, such that a radiation dose delivered to an object during a medical X-ray image recording operation may be kept small, and such that the image quality of an image and its capability of serving as an effective tool in, particularly, the efficient and accurate diagnosis of an illness may be enhanced, a photo-conductive material sensitive to X-rays, such as a selenium plate constituted of, e.g., amorphous selenium (a-Se), is employed as an electrostatic recording material. The electrostatic recording material is exposed to radiation (i.e., recording light), such as X-rays, carrying radiation image information, and latent image charges carrying the radiation image information are thereby accumulated in the electrostatic recording material. Thereafter, the electrostatic recording material is scanned with reading light, such as a laser beam, and currents occurring in the electrostatic recording material are detected via flat plate-shaped electrodes or stripe-shaped electrodes, which are located on opposite surface sides of the electrostatic recording material. In this manner, the electrostatic latent image represented by the latent image charges, i.e. the radiation image information, is readout. (Such systems for recording and reading out radiation image information are described in, for example, Patent Literature 1, Patent Literature 2, Patent Literature 3, and Non-Patent Literature 1.)
Also, the applicant proposed an electrostatic recording material and a read-out apparatus for reading out radiation image information from the electrostatic recording material, on which the radiation image information has been recorded. The proposed electrostatic recording material comprises:
i) a first electrical conductor layer having transmissivity to recording radiation,
ii) a recording photo-conductive layer, which exhibits photo-conductivity when it is exposed to the recording radiation,
iii) a charge transporting layer, which acts approximately as an insulator with respect to electric charges having a polarity identical with the polarity of electric charges occurring in the first electrical conductor layer, and which acts approximately as a conductor with respect to electric charges having a polarity opposite to the polarity of the electric charges occurring in the first electrical conductor layer,
iv) a reading photo-conductive layer, which exhibits photo-conductivity when it is exposed to reading light, and
v) a second electrical conductor layer having transmissivity to the reading light,
the layers being overlaid in this order. (The proposed electrostatic recording material and the proposed read-out apparatus are described in, for example, Patent Literature 4.)
With the read-out apparatus described in Patent Literature 4, the electrostatic recording material, on which an electrostatic latent image has been recorded, is scanned with the reading light having been produced by a light source, and the electrostatic latent image having been recorded on the electrostatic recording material is thereby read out. The exposure scanning with the reading light may be performed with a technique, wherein the electrostatic recording material is scanned with spot light, such as a laser beam, in a main scanning direction and a sub-scanning direction, a technique, wherein a line light source for producing linear reading light is moved in a sub-scanning direction, or the like.
The conventional read-out apparatus described above is often utilized for medical purposes. In such cases, it is necessary for the dose of X-rays, which are ordinarily employed as the recording light, to be kept small. Therefore, it is required that the read-out apparatus be a low-noise system. Also, in order for adverse effects of X-ray scattering to be suppressed, a grid is often utilized at the time of the image recording. In order for moire stripes to be prevented from occurring, a moving grid, which vibrates, is often utilized at the time of the image recording. (The technique for utilizing the moving grid is described in, for example, Patent Literature 6.)
Patent Literature 1:                U.S. Pat. No. 4,176,275        
Patent Literature 2:                U.S. Pat. No. 5,440,146        
Patent Literature 3:                U.S. Pat. No. 5,510,626        
Patent Literature 4:                U.S. Pat. No. 6,268,614        
Patent Literature 5:                U.S. Patent Laid-Open No. 20010025936        
Patent Literature 6:                U.S. Patent Laid-Open No. 20020001366        
Non-Patent Literature 1:                “A Method of Electronic Readout of Electrophotographic and Electroradiographic Image,” Journal of Applied Photographic Engineering, Volume 4, Number 4, Fall 1978, pp. 178-182        
However, in the cases of electrostatic latent image read-out apparatuses provided with a moving member, which moves or has possibility of moving while the exposure scanning with the reading light is being performed, and the signal is being readout, it often occurs that the moving member constitutes a noise source, and that a signal-to-noise ratio of the signal having been read out becomes low. Specifically, in cases where the moving member has the characteristics of undergoing capacity coupling with the electrostatic recording material, if the moving member moves at the time of the exposure scanning, the problems will occur in that the capacity of the electrostatic recording material fluctuates, and that the signal-to-noise ratio of the signal obtained from the exposure scanning becomes low.
In cases where the exposure scanning is performed through the movement of the reading light source, it often occurs that the reading light source itself acts as the moving member that constitutes the noise source. Also, a releasably secured member which is not secured firmly, or the like, has the possibility of moving at the time of the exposure scanning, and it may be considered that the releasably secured member, or the like, is one kind of the moving member that constitutes the noise source. In particular, in the cases of the read-out apparatuses, in which the reading light source moves during the exposure scanning, due to the moving operation of the reading light source, there is the possibility that, besides the reading light source itself, other moving members will move.
Also, in Patent Literature 6 described above, the moving grid is described as one of the moving members. Specifically, in cases where the outputting of the signal is performed in the state in which the vibrations of the moving grid have not yet been ceased perfectly, it often occurs that the fluctuation in capacity of the electrostatic recording material occurs, and that the signal-to-noise ratio of the obtained signal become low. Therefore, it may be considered that the moving grid also acts as the moving member that constitutes the noise source. In Patent Literature 6 described above, a technique for operating the moving member at uniform speed, or a technique for performing the outputting of the signal after the movement of the moving member has been ceased perfectly is utilized in order to prevent the capacity of the electrostatic recording material from fluctuating at the time of the outputting of the signal. However, with the technique utilized in Patent Literature 6 described above, the problems occur in that a throughput of the signal outputting operation cannot be kept high.